To measure a position of an underwater vehicle (UUV (Unmanned Underwater Vehicle) or the like) which moves in the water is carried out. To measure the position includes to measure a distance (distance measurement) to a target, etc. In general, a sound signal is used for measuring the position. The reason is that the sound signal has smaller propagation loss in the water than another signal such as an electromagnetic wave signal has.
The distance measurement using the sound signal is carried out by measuring a propagation time (refer to NPL 1). Therefore, equipment, which is called the responder, is installed in the vehicle. The responder receives a sound signal which is sent by a target, and sends back a sound signal to the target. Then, a propagation time, which is defined as a period of time from the target's sending the sound signal until the target's receiving the sound signal which is sent back, is measured, and a distance between the target and the vehicle is calculated by use of the propagation time and a propagation speed of the sound signal.
Moreover, to develop an art, which uses not only the sound signal but also an electromagnetic wave signal for the distance measurement, has been started. For example, PTL1 discloses an art that, on the basis of a difference between a propagation time taken when a sound signal propagates from a point 1 to a point 2, and a propagation time taken when an electromagnetic signal propagates from the point 1 to the point 2, a distance between the point 1 and the point 2 is calculated.
Moreover, as a related art which uses the electromagnetic wave signal, PTLs 2 to 4 disclose an art that the electromagnetic wave signal is used for information transmission in the water.